1. Field of the Invention
This invention relates to the use of submicron boron nitride as the catalyst in producing a polysiloxane resin formulation and the controlled processes to provide glass fabric and filler reinforced composite gaskets from the resin formulation.
2. Description of the Previously Published Art
A variety of polysiloxane oligimers that are well known in the art (currently supplied by Dow Corning and GE Silicones and others) can be used to formulate polysiloxane resins containing catalyst and additives for composite processing. Conventional curing of polysiloxane resin formulations is well documented in the literature. See for example U.S. Pat. Nos. 4,927,587 and 5,552,466. The polysiloxane blends employ conventional means such as chemical curing or heat curing.
Typical catalysts used to crosslink silicone polymers include condensation catalysts. Depending upon the silicon polymer's reactive groups, other catalysts and initiators can also be employed. Free radical catalysts such as peroxide catalysts may be used when silicone polymers contain a vinyl group. Ultraviolet light radiation and silane-olefin addition (hydrosilation) may be used when silicone polymers have terminal double bonds or silicone hydride groups in the terminal positions. Silicone compounds with hydroxy groups can also be catalyzed with heat.
In these processes (U.S. Pat. No. 5,552,466 and Preparative Methods of Polymer Chemistry, W. R. Sorenson et al., John Wiley & Sons, pp. 383-390 (1961) and Inorganic and Organometallic Polymers "Recent Advances In Organosiloxane Copolymers", J. D. Summers et al., American Chemical Society, pp. 180-197) described for forming a resin blend of polysiloxane, there has been some mention of boron nitride as a filler in combination with the above described typical catalysts. However the method of controlling the polymerization is with conventional catalysts or heat. Most prior art has concentrated upon quartz and related ceramic additives for enhancing the ceramic yield while using conventional catalysts for controlling the polymerization process.
3. Objectives of the Invention
Boron nitride when used in combination with a conventional catalyst such as zinc hexanoic acid (U.S. Pat. No. 5,552,466), has been observed to also catalyze the reaction. This approach was observed by the inventor to become uncontrollable as the preferred submicron size of the boron nitride is employed. Essentially, the time to prepare a hot melt impregnation polysiloxane resin solution using the combination of catalysts is too short for practical "prepreg" processing and resulted in a run away reaction at the preferred concentration of the conventional catalyst. The preferred amount (U.S. Pat. No. 5,552,466) of the organo-zinc catalysts is 0.1 to 0.5 percent by weight of the resin blend, consequently, a further reduction in this already low level of catalyst is not desirable and more importantly, the boron nitride is preferred at higher concentrations and with submicron size for producing uniform properties. The preferred boron nitride properties are high temperature lubrication of glass fibers which enhance the high temperature strength and thermal conductivity of glass fabric reinforced high temperature non-metallic composite gaskets.
It is the objective of this invention to allow polysiloxane polymer blends to be processed with submicron boron nitride particulate as the controlling catalyst without the additional use of conventional catalysts. The boron nitride submicron particulate is inherently porous, consequently its surface area is significantly greater than non-porous submicron particles. Also, the weight ratio of resin to (submicron) boron nitride is most practical between a 5 to 1 and 20 to 1 range. This weight range allows the preferred polysiloxane blend to be controlled within practical "gel" (gelation) time at 350.degree. F. (177.degree. C.) limits of 2 minutes to 10 minutes when processed. The gel time for boron nitride (taken at different concentrations of the catalyst) is shown in FIG. 1 for a preferred polysiloxane formulation. The gel can be observed precisely as the time when the polymerizing mixture suddenly loses fluidity while constantly stirring at 350.degree. F. (177.degree. C.), e.g., when bubbles no longer rise in it.
It is the further objective of this invention to provide a practical method of processing the preferred polysiloxane resin blend into hot melt impregnation blends with a high quality uniform dispersion of the submicron boron nitride catalyst. Submicron boron nitride tends to "clump" together when added to polysiloxane blends which results in "streaking" during "prepreg" processing. This condition also renders the boron nitride not practical as an effective controlling catalyst because it diminishes the available surface area. The preferred processing approach is to disperse the submicron boron nitride into the preferred polysiloxane blend using anhydrous acetone to facilitate an even distribution with minimal to no submicron clumping observed. The use of anhydrous acetone allows the ease of acetone removal and thoroughly dissolves all polysiloxane blend constituents without leaving water contamination.
It is the further objective of this invention to produce a variety of high temperature non-metallic composite gaskets. The gaskets are cut from flat composite laminates molded from various ceramic fabrics and fiber reinforcements with a preferred S-glass, 8HS, style 6781 fabric and impregnated with the preferred polysiloxane formulation loaded with boron nitride and quartz filler. The preferred prepreg produced from hot melt processing is catalyzed with boron nitride within the concentration range given in FIG. 1. This invention allows optimal control of submicron boron nitride loaded polysiloxane resin blends for hot melt prepreg processing.